The invention concerns lead-acid batteries and, more specifically, the design of the positive electrodes in such batteries.
Lead-acid batteries typically comprise positive and negative electrode plates separated by microporous separators, with sulfuric acid serving as an electrolyte. During discharge, as many sulfate ions are used at the positive electrode as at the negative electrode. However, at the positive electrode water is generated and the porosity decreases more than at the negative electrode, i.e., less surface area of the positive electrode is available to admit sulfuric acid. Problems at the negative electrode are minimized by the fact that the sulfate ions have a potential which urges them toward the negative electrode.
The availability of sulfuric acid at the positive electrode is further limited by the fact that acid transport in the microporous separator is difficult, and thus the travel of acid to the positive electrode is resisted.
In order to attempt to compensate for the limited availability of sulfuric acid at the positive electrode, the separators have heretofore been provided with ribs facing the positive electrode, which ribs define channels or pockets for confining sulfuric acid. Hence, access of the acid to the positive electrode is somewhat improved.
In spite of this arrangement, the capacity of the cell continues to be limited by the positive electrode due to the restricted access of acid to the positive active material.
To increase capacity, the positive electrode can be made thinner and more porous, thereby facilitating travel of the acid thereto. The use of thinner plates, however, requires more electrodes and separators to maintain capacity and thus involves higher costs and also thinner current collectors with higher corrosion and shorter life as a result. The higher porosity also decreases the life of the cell due to rapid loss of active material from the positive electrode.
Those problems are complicated by the fact that the life of the positive electrode is a function of the loss in volume of the active material. In high quality batteries, such as industrial batteries, for example, porous material such as glasswool is used to support the surface of the positive electrode during regular discharge to help retain the volume thereof. Such material, however, further decreases the rate of acid transport. In certain designs, the ribs on the separators are replaced with thicker or thinner layers of porous material such as glasswool.
One design of a positive electrode heretofore proposed to support the active material and improve the life span of the positive electrode involves a so-called tubular plate comprising a number of parallel tubes. Each tube comprises a current collector imbedded within and contacting the active material. The collectors are joined together by a bar at the top. If the tubes in such a tubular plate are made of a material such as woven or braided glass which can withstand the pressure occurring during discharge, it is possible to get very small volume changes and thus obtain a good life span for the tubular plate.
Tubes have been proposed which in one case are of circular shape and, in another case are of square shape. A plurality of mutually contacting circular tubes offers increased surface area and thus increased porosity (see Swiss Pat. No. 199,710 published Nov. 16, 1965). The small area between adjacent tubes contains acid which penetrates the positive active material. However, such a design of a positive electrode results in an increased ratio of volume to surface area which can be disadvantageous. Moreover, the small space between the round tubes is insufficient to adequately supply the positive electrode, thereby requiring the use of reservoir-defining ribs on the separator.
Even if round tubes are spaced apart so as to provide additional space therebetween, the capacity of the cell can only be maintained by adding additional plates and separators in lieu of the ribs, but this significantly increases the cost of the accumulator.
As mentioned above, square-shaped tubes have been proposed in order to increase the surface area of the tubular plate and thereby facilitate the penetration of acid into the positive active material. There exists between the square tubes a slight incidental spacing of about 1 mm, whereby a minimal amount of acid can become situated between the tubes. Ribbed spacers are still required, however, to provide acid for the positive electrode.
Moreover, as the active material in the electrodes swells during discharge, there occurs a pressure in the tubes which the square shape cannot withstand, with the result that the walls of the tubes expand and contact each other, even after perhaps only some ten discharges. Since a high quality accumulator shall have a life of 1000-2500 discharges, there will be almost no free acid between the tubes in such design; but rather will be in the pores of the walls and electrode and between the positive and negative plate and at the side of and under the plate group.
It has also been suggested to provide tubes with somewhat concave side surfaces which define electrolyte channels therebetween (see German Pat. No. 961,720). The tubes are formed of plastic and include horizontal slots for admitting electrolyte. However, the capacity of the accumulator is very limited since contact between electrolyte and active material is possible only by means of the spaced slots.
As noted earlier, it is desirable to minimize the ratio of volume-to-surface area of the positive electrode tube. For a square or circular tube, such a ratio is 0.25 d, where d is the diameter of a circular tube or a side dimension of a square tube. Thus, it would be desirable to provide a positive electrode tube whose ratio of volume to surface area is less than 0.25 d while maximizing or maintaining the capacity of the electrode.
Presently used tubes exhibit very limited outer support, making it necessary to form them of strong material-like braided or woven continuous fibers to enable the tubes to withstand the pressure developed therewithin. It would be desirable to eliminate such an expensive tube construction.
When a lead acid battery is charged following a discharge, oxygen begins to develop when only 80-90% of the battery is charged, whereas hydrogen does not evolve until after 98-100% of the previous discharge. It has been found that if the positive and negative electrodes can be pressed together with only a thin hydrous separator between them, the evolving oxygen will be forced through the separator into the negative plate. If the negative plate is oxydized in this way, the battery can be fully charged till 100% without any gas evolution at all. Such sealed, high-pressure batteries have recently been commercialized. However, as recombination is related to the pressure between plates and the thickness of the separator, such batteries are starved for electrolyte which are available only in the pores of the plate or in the thin separator. It would be desirable to enable the pressure to be maintained within such batteries while providing ample electrolyte to the plates.
It is, therefore, an object of the present invention to minimize or obviate problems of the type noted above.
It is another object of the present invention to provide an electrical accumulator having increased capacity and unchanged volume.
It is a further object of the invention to provide an electrical accumulator in which the spacing between the positive and negative active material is effectively minimized.
One further object of the invention is by outer support of the tubes to be able to use very inexpensive material for the tube walls and in this way considerably reduce the cost of the tubes.
It is a further object of the invention to provide an electrical accumulator of the sealed, pressurized type wherein adequate pressure is maintained and an ample electrolyte supply is provided.
It is still a further object of the invention to provide a positive electrode tube having a minimized ratio of volume-to-surface area.
It is an additional object of the invention to provide a novel method for fabricating electrode tubes of minimized volume-to-surface area ratio and maximized support between tubes.